1. Field of the Invention
The present invention relates to epoxy resin compositions suitable for forming anisotropically conductive adhesive films, to anisotropically conductive adhesive films formed from the epoxy resin compositions, and to an electrical connecting method between conductors using the anisotropically conductive adhesive films.
2. Description of the Related Art
A variety of conventional epoxy resin compositions are well-known, and many efforts have been made toward improving their characteristics.
For example, styrenic thermoplastic elastomers (hereinafter referred to as "styrenic elastomers") are added to improve the impact resistance of epoxy resin compositions. Addition of styrenic elastomers can improve the impact resistance of epoxy resins across a wide temperature range, alleviate residual internal stress caused by the curing reaction, and improve the adhesion reliability. Such epoxy resin and styrenic elastomer compositions are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 8-20629, No. 7-166145, No. 4-370137 and No. 49-20 25039. However, since the compositions disclosed in these publications do not involve reaction between the styrenic elastomer and epoxy resin, the cured compositions have low heat resistance and moisture resistance, and thus insufficient adhesion reliability. Also, since epoxy resins and styrenic elastomers normally have low compatibility, there are limitations to the mixing ratio if a uniform composition is desired.
In addition, Japanese Unexamined Patent Publication (Kokai) No. 7-197000, No. 4-224818 and No. 4-91183 disclose curing compositions containing epoxy resins and acid-modified styrenic elastomers which can react with those resins. The use of acid-modified styrenic elastomers in these curing compositions provides improved compatibility between the styrenic elastomer and the epoxy resin. Nevertheless, progressive reaction between the acidic functional groups and epoxy resin during storage of these compositions tends to shorten their usable life. The curing agents included in these compositions cause crosslinking by polyaddition reaction, the reaction temperature is at least 150.degree. C. and the reaction time is relatively long, at a half hour or more. They are therefore unsuitable as adhesive materials to be used in the fields of electricity or electronics, which require high-speed curing, or curing within a very short time (for example, within 1 minute).
An epoxy resin composition comprising an epoxy resin and a reactive styrenic elastomer, which is used to form an anisotropically conductive adhesive film (hereinafter referred to as "conductive adhesive film") is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-32799 (Sumitomo Bakelite Co.), which uses a microencapsulated imidazole derivative as a curing agent. This conductive adhesive film provides sufficiently strong adhesion between microcircuit boards such as FPCs (flexible printed circuits), while establishing electrically continuous connections between conductors such as connecting terminals on mutually facing substrates without shorting out the circuits. Such conductive adhesive films are formed by dispersing conductive particles in an insulating adhesive such as an epoxy resin and forming a film. Connections between conductors are usually established with the conductive adhesive film in the following fashion. After inserting the adhesive film between the two substrates, pressure and heat are applied to complete the adhesion. Thus, since the adhesion is achieved between the mutually facing connecting terminals with the conductive particles in an electrically continuous state along the direction of thickness of the film (usually called the "Z-axis direction"), continuity is established between the facing connecting terminals.
However, in order to meet recent demands for improved productivity, it has been necessary to complete adhesion with a very short adhesion time, specifically by thermal contact bonding for 10-30 seconds. As one means of meeting this demand it has been proposed to use microencapsulated imidazole derivatives as curing agents, as in Japanese Unexamined Patent Publication (Kokai) No. 5-32799 above. This presents the risk of capsule rupture due to thermal and mechanical factors during the production process, and therefore this proposal is not advantageous from a production standpoint.
Thus, conventional conductive adhesive films using epoxy resins and styrenic elastomers have not been able to provide improvement in all the performance parameters of speed-curing, heat and moisture resistance and adhesion reliability.
Addition of cationic polymerization initiator catalysts has also been considered for application to high-speed curable epoxy resin adhesives. Lewis acids and their coordinated compounds can generally be used as cationic polymerization initiator catalysts, and therefore higher reactivity and higher speed curing can be achieved by their combination with alicyclic epoxy resins than with glycidyl ether-type epoxy resins. However, combinations of alicyclic epoxy resins and common cationic polymerization initiator catalysts have not been practical because of their short usable life.
Attention has therefore turned to "ultraviolet active cationic polymerization catalysts" (hereinafter referred to as "UV catalysts"), which have low catalytic activity in the absence of ultraviolet irradiation but increase their activity as catalysts upon ultraviolet irradiation. That is, it has been suggested that better storage stability (longer usable life) can be achieved by using such UV catalysts. Documents dealing with UV catalysts include H. J. Hageman, Progr. Org. Coat. 13, 123 (1985), and European Patent Application No. 0094915 (1984).
Also, U.S. Pat. No. 5,362,421 (corresponding to Japanese National Publication (Kohyo) No. 8-511570) discloses acceleration of epoxy resin curing reactions by cationic polymerization reaction using alcohols, including diols. However, to date, no epoxy resin composition has been provided which includes a combination of an alicyclic epoxy resin and a UV catalyst and has sufficient impact resistance.
Another approach for providing conductive adhesive films is to improve the low temperature curability (allow curing at low temperature). In recent years, plastic base liquid crystal displays (LCDs), and flexible printed circuits (FPCs) using polyethylene terephthalate (PET) films as base materials have been developed in order to lower costs and decrease the weight of liquid crystal (LCD) panels. Consequently, establishing these connections at contact bonding temperatures for known conductive adhesive films (150-200.degree. C.) results in thermal damage to the LCDs and FPCs. In addition, the high flexibility of FPCs is not able to alleviate the stress produced by the difference in heat expansion between the mutually adhered circuit boards, contact bonding at high temperatures causes large deformation of the film base materials and creates "wrinkles" on the FPCs. This stress alleviation problem also needs to be solved with connections between the glass panels of the LCDs and the FPCs employing polyimide film base materials. From this standpoint, a contact bonding temperature of 120.degree. C. or below is usually required.
Examples of conductive adhesive films which can be cured at low temperature are disclosed in Japanese Unexamined Patent Publications (Kokai) No. 4-189883 and No. 7-90237. Both of the conductive adhesive films disclosed in these publications have improved curing agents for the epoxy resins, and have peak activation temperatures near 100.degree. C., as measured using a DSC (differential scanning calorimeter), with a temperature elevating rate of 10.degree. C./min. When adhesion is actually carried out in a relatively short time at a contact bonding temperature of 120.degree. C. or below, the peak activation temperature must be less than 100.degree. C. in order to achieve sufficient adhesion reliability. Also, since the highly reactive curing agent is in admixture with the epoxy resin during storage, there is also a risk of reduced storage stability.
Japanese Unexamined Patent Publication (Kokai) No. 8-111124 discloses a conductive adhesive film with a 3-layered structure of a thermosetting resin layer, a separating layer and a curing agent layer. This conductive adhesive film has the highly reactive curing agent separated from the thermosetting resin, thus achieving excellent storage stability while also providing a peel strength of 1000 g/cm during contact bonding at 100.degree. C. for 20 seconds. Still, the formation of the three layers by application methods requires a complex production process, while coating defects in separating layers can result in reduced storage stability.
As explained above, conventional epoxy resin compositions and conductive adhesive films formed using them have a large number of problems.
It is therefore an object of the present invention to overcome these problems by providing an epoxy resin composition suitable for adhesive materials used in the fields of electricity and electronics, such as conductive adhesive films, specifically an epoxy resin composition which provides enhancements and improvements in all of the following: (i) high-speed curing, (ii) heat and moisture resistance, (iii) shock resistance, (iv) storage stability and (v) low temperature curability.
It is another object of the invention to provide conductive adhesive films which employ the epoxy resin composition of the invention.
It is yet another object of the invention to provide a process for establishing electrical connections between conductors using conductive adhesive films according to the invention.